166 CARBONIFEROUS LAMELLIBRANCHIATA. 



covered with numerous regular, close-set, concentric stria?, which follow the 

 contour of the shell, and are divided into groups by the deeper lines. There is no 

 trace of any radiating striae. 



Dimensions. — PI. XII, fig. 10, measures — 



Antero-posteriorly. Dorso-ventrally. Elevation of valve. 



(Restored) 30 mm. 20 mm. 10 mm. 



Localities. — The Carboniferous Limestone of Settle, Yorkshire; Castleton and 

 Thorpe Cloud, Derbyshire. 



Observations. — A rare form, somewhat resembling P. decussatus, but more 

 gibbose, and with no radiating lines. 



I have referred the shells figured PI. XII, figs. 8, 10, and 13, to P. Fraiponti, 

 de Koninck, to which species I think they have the greatest resemblance. It is 

 difficult to be completely sure without comparing the specimens side by side, and 

 it is now several months since I examined the types at Brussels. The description 

 given by de Koninck for the P. Fraiponti corresponds in every particular with 

 the shells I have for description. 



He lays stress on the smooth surface and occasional deep concentric marking, 

 but states in his remarks that fine concentric striae are absent, and adds that the 

 hinge consists of three or four long posterior teeth, and the same number of 

 oblique teeth in front. I think it probable that P. latus of this author is only 

 another example of the species under discussion. They both have the same 

 surface ornament, are closely similar in shape, and occur at the same horizon at 

 Vise, where both are stated to be very rare. De Koninck's largest example of 

 P. Fraiponti measures 53 mm. transversely, 34 mm. dorso-ventrally, and 16 mm. 

 in thickness, but it is not stated if the latter is the depth of a single or both valves. 



In shells which have not come out of the matrix very perfectly the outer 

 layers of the test are often left behind, and therefore the microscopic concentric 

 lines are only seen in the very best preserved examples, which will perhaps 

 account for the fact that de Koninck so pointedly remarks on their absence. 



The ornamentation of the valve by groups of fine lines, separated by more 

 strongly marked striae at unequal distance, is that which obtains in P. Haimeanus, 

 but the strong lines are more numerous in the latter, and the shapes of the shell 

 are very different in the two species. 



I figure a fine example of P. Fraiponti on PI. XII, fig. 10, from the 

 Carboniferous Limestone of Settle, which is in the York Museum. It has 

 unfortunately lost the anterior portion of the test, which is shown to be 

 moderately thick, far thicker than in any other shells of the same species which I 

 have obtained from other localities. 



Though not common, this species seems to have a tolerably wide distribution 

 in the Upper Beds of the "Massif" of Carboniferous Limestone. 



